REVIEW 3 major objections 5 minor 29 references
Edge2Prompt: Modality-Agnostic Model for Out-of-Distribution Liver Segmentation
T0 review · 3 major / 5 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read Edge2Prompt claims a single pipeline segments liver across CT and MRI by prompting SAM-2 with logits derived from edge maps.
desk verdict Reasonable idea, but the headline OOD gain is against the weakest baseline and the data-scarce test set doesn't add up; deserves review after fixing the protocol. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
Edge-to-prompt transfer: histogram-equalized Canny edge maps are fed into a modified U-Net whose output logits serve as the prompt for a frozen SAM-2, connected through a differentiable wrapper so gradients flow back to the U-Net. The logit prompt replaces manual point- or box-based prompting, the binarized edges strip away intensity-based modality cues, and the frozen SAM-2 supplies the shape prior and refinement.
What would settle it
Train Edge2Prompt on MR T1-DUAL only. On CT test slices, corrupt the edge maps by randomly dropping half the boundary pixels or flipping edge/non-edge labels. If out-of-distribution Dice stays near 85%, the edge-to-prompt mapping is not what carries the result; if it collapses toward the U-Net baseline of about 62%, edge-map fidelity is the mechanism.
Extended reading notes
Core claim
The central claim is that binarized Canny edge maps, extracted after histogram equalization, preserve enough liver-boundary structure across CT and MRI that a single U-Net can learn transferable logit prompts for SAM-2. Edge2Prompt is trained end-to-end while all SAM-2 parameters stay frozen; the composite loss of focal, dice, and log-cosh dice terms makes the prompt-generating U-Net learn a task that SAM-2 can refine. With training on a single CT and a single MR volume, or on one modality alone, the pipeline reconstructs coherent 3D liver volumes from segmented slices and outperforms standard U-Nets and a coarse-mask self-prompting baseline on out-of-distribution data.
Load-bearing premise
The whole out-of-distribution gain rests on one premise: that binarized Canny edges preserve the same liver boundary structure in CT and MRI, so a U-Net trained on one modality's edges can prompt SAM-2 for the other.
Editorial extensions
If this is right
- One trained model can segment livers in both CT and MRI without retraining, because edge maps remove the intensity and contrast cues that differ between modalities.
- Training on a single CT and a single MR volume is enough for usable out-of-distribution segmentation, which directly addresses annotation scarcity in clinical deployment.
- SAM-2 stays frozen during training, so the pipeline does not need to fine-tune a large foundation model and remains cheap to deploy and update.
- Segmented 2D slices can be stacked into 3D volumes via an affine transform, giving clinicians reconstruction-ready output for preoperative planning.
- The method outperforms U-Net baselines and another self-prompting SAM-2 approach on cross-modality generalization, suggesting the edge-prompt path is more robust than raw-image or coarse-mask prompting.
Reading between the lines
- The ablation showing Sobel edges nearly matching Canny suggests the exact edge detector is not the load-bearing choice; what matters may be that the edge map is binarized and topology-preserving. A simpler binary boundary operator could work equally well.
- If edge maps are truly the modality-agnostic carrier, the same prompt-generating U-Net should transfer to other organs or to ultrasound and fluoroscopy with only small retraining; the paper does not test this, but it is a direct consequence of the stated mechanism.
- Because the U-Net output is a logit prompt rather than a final mask, the model may be learning a task-specific prior that SAM-2 refines; this predicts that failures will concentrate on small or faint liver regions, which is consistent with the high variance the paper reports.
- A testable extension: corrupting edge maps at test time (randomly dropping or flipping boundary pixels) should degrade out-of-distribution Dice much more than in-distribution Dice, since cross-modality transfer depends on boundary consistency.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes Edge2Prompt, a segmentation pipeline that combines classical edge detection with a trainable U-Net and a frozen SAM-2 model. Given a 2D grayscale CT or MR slice, an Edge Detection Module extracts a Canny edge map, a U-Net converts that edge map into logit-based prompts, and SAM-2 uses those prompts to produce a refined liver segmentation. Segmented slices are then stacked into a 3D volume. The method is evaluated on the CHAOS dataset under three scenarios: in-distribution mixed-modality training, a data-scarce setting with two training volumes, and cross-modality out-of-distribution generalization (train on CT, test on MR and vice versa). The authors report a mean OOD Dice of 86.4%, claiming a 27.4% improvement over U-Net baselines and a 9.1% improvement over another self-prompting SAM-2 method.
Significance. If the reported results are reproducible, the paper makes a useful empirical contribution: it demonstrates that a lightweight prompt generator trained on cheap edge maps can condition a frozen foundation model to segment the liver across CT and MRI with little training data. The end-to-end differentiable wrapper around frozen SAM-2, the composite loss ablation, and the systematic comparison against imU-Net, emU-Net, sU-Net, and spSAM are strengths. The method does not require fine-tuning SAM-2, which is practical for clinical settings with limited compute. However, the main quantitative claims rest on an under-specified data split and a selective baseline comparison. The data-scarce results in Table 1 are not verifiable from the described partition, and the abstract's headline margin is computed against only the weakest U-Net baseline. These issues are fixable but must be addressed before the central claims can be accepted.
major comments (3)
- [§3.2 / Table 1] The evaluation protocol is numerically inconsistent. The authors state D_TRAIN_T = 20 volumes (10 CT + 10 MR), D_VAL_T = 10, and D_TEST_T = 10, which accounts for all 40 CHAOS volumes. They then introduce D_TRAIN_J = 1 CT + 1 MR and D_TEST_J = 14 CT + 14 MR. These 28 test volumes cannot be formed from the stated partition: after removing D_TRAIN_J, only 19 CT + 19 MR volumes remain, and the original validation/test sets contain only 10 CT + 10 MR combined. If D_TEST_J includes volumes from D_TRAIN_T or D_VAL_T, then the Table 1 rows are not comparable and the data-scarce results may reflect a different, potentially easier, test distribution. Please disclose the exact volume-level split, including patient/scan identifiers, and rerun or re-report the data-scarce evaluation on a held-out set that does not overlap with the volumes used elsewhere.
- [Abstract / §4.1 / Table 2] The claim of 'outperforming U-Net baselines by 27.4%' is an average over only imU-Net, the weakest baseline. From Table 2, the OOD Dice differences are CT: 87.6−62.2 = 25.4 and MR: 85.3−55.9 = 29.4, whose mean is 27.4. Against emU-Net, the U-Net baseline that also uses edge maps, the average improvement is (87.6−80.2 + 85.3−75.2)/2 = 8.8 percentage points. Since the abstract refers to 'U-Net baselines' in the plural, the current wording materially overstates the method's advantage over the stronger edge-based baseline. Please report margins against each baseline separately and adjust the abstract.
- [§2.1 / Table 3b] The central methodological premise is that Canny edge maps after histogram equalization are 'modality-agnostic' and preserve transferable liver-boundary structure across CT and MRI. This invariance is never directly tested. The ablation in Table 3b shows Sobel and Canny produce nearly identical OOD Dice (85.2 vs 85.3), which suggests the specific edge detector is not the source of robustness; it does not establish that edge maps are modality-agnostic. I recommend adding a direct analysis, e.g., a qualitative/quantitative comparison of edge maps from co-registered or corresponding CT/MR slices, or an ablation that replaces edge-map input with raw intensity input in the same SAM-2-conditioned architecture (the comparison with sU-Net is suggestive but sU-Net also differs in U-Net input). Without this, the mechanistic explanation for the OOD gain remains a conjecture.
minor comments (5)
- [§3.2] 'T1-DUAL OOP' is ambiguous. Earlier the paper mentions 'T1-DUAL (In-Phase/Out-Phase)'; please define 'OOP' explicitly (presumably out-of-phase).
- [§4.1] The claim of '+14.4% mDICE' over spSAM on ID tasks does not match the numbers in Tables 1 and 2. Table 1 gives 95.1−78.8 = 16.3; Table 2 gives an average of (95.1−77.8 + 92.8−81.1)/2 = 14.5. Please clarify how 14.4% was computed.
- [§2.3] The volumetric reconstruction section mentions interpolation or super-resolution for sparse MR slices, but no quantitative evaluation of the reconstruction quality is provided beyond two visual cases. This is acceptable as a secondary contribution, but a sentence stating that the 3D reconstruction is illustrative rather than quantitatively validated would help.
- [General] The source code is only announced as 'will be made publicly available.' For reproducibility, provide a code link or an artifact in the submission.
- [Table 3b] The Laplacian detector has a very high standard deviation (69.6 ± 37.2 Dice). It would be informative to report the number of failing cases (e.g., near-zero Dice) to help interpret the variance.
Circularity Check
No significant circularity; Edge2Prompt's reported segmentations are genuine model outputs, and the data-split / model-selection concerns are validity issues, not derivation equivalences.
full rationale
Edge2Prompt is an empirical machine-learning pipeline, not a derivation from first principles. The only 'prediction' is the 2D/3D liver segmentation, produced by a frozen SAM-2 conditioned on U-Net logits, and evaluated on the CHAOS dataset against external and internal baselines. I could not find any equation, definition, or fitting operation that makes the reported Dice scores equal to an input or to a self-citation by construction. Loss weights (wF=2, wD=2, wLCD=3) and the Canny edge detector are chosen empirically, but they are hyperparameters rather than parameters fitted to the test labels; the reported segmentation is still an independent function of the input image. The ablation in Table 3a selects the best loss configuration on the OOD test task, which is an optimistic model-selection practice, but it does not make the OOD result an identity. The only author self-citation (Ref. [11], by co-author S. El hadramy) appears as background in the introduction and is not load-bearing for any technical claim. I also note a reproducible-validity issue outside circularity: the §3.2 partition is internally inconsistent—D_TRAIN_T (10 CT + 10 MR), D_VAL_T (5 CT + 5 MR), and D_TEST_T (5 CT + 5 MR) account for all 40 volumes, so a later D_TEST_J of 14 CT + 14 MR after removing a 1+1 training subset cannot be derived from the stated split. This undermines the data-scarce benchmark clarity but is not a circularity of the method's derivation chain. Overall, no circular step was found.
Assumptions & free parameters
free parameters (2)
- Canny edge detector thresholds and sigma =
not specified in paper
- Composite loss weights wF, wD, wLCD =
wF=2, wD=2, wLCD=3
assumptions (3)
- domain assumption Canny edges from histogram-equalized images are modality-agnostic across CT and MR
- domain assumption SAM-2 can refine liver masks from U-Net logit prompts without fine-tuning
- domain assumption CHAOS dataset is representative of clinical CT/MR liver imaging
Cite this review
Pith. "Pith review of Edge2Prompt: Modality-Agnostic Model for Out-of-Distribution Liver Segmentation." pith.science (2026). https://pith.science/paper/ORGTEXZO
@misc{pith2026250804305,
author = {Pith},
title = {Pith review of: Edge2Prompt: Modality-Agnostic Model for Out-of-Distribution Liver Segmentation},
year = {2026},
howpublished = {\url{https://pith.science/paper/ORGTEXZO}},
note = {Machine review of arXiv:2508.04305}
}
read the original abstract
Liver segmentation is essential for preoperative planning in interventions like tumor resection or transplantation, but implementation in clinical workflows faces challenges due to modality-specific tools and data scarcity. We propose Edge2Prompt, a novel pipeline for modality-agnostic liver segmentation that generalizes to out-of-distribution (OOD) data. Our method integrates classical edge detection with foundation models. Modality-agnostic edge maps are first extracted from input images, then processed by a U-Net to generate logit-based prompts. These prompts condition the Segment Anything Model 2 (SAM-2) to generate 2D liver segmentations, which can then be reconstructed into 3D volumes. Evaluated on the multi-modal CHAOS dataset, Edge2Prompt achieves competitive results compared to classical segmentation methods when trained and tested in-distribution (ID), and outperforms them in data-scarce scenarios due to the SAM-2 module. Furthermore, it achieves a mean Dice Score of 86.4% on OOD tasks, outperforming U-Net baselines by 27.4% and other self-prompting methods by 9.1%, demonstrating its effectiveness. This work bridges classical and foundation models for clinically adaptable, data-efficient segmentation.
Figures
Reference graph
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Reviewed August 6, 2026 · model on record in the stance chip above.
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